Technically, you can't just "convert" a closed file on disk. It has to be opened and rewritten to the new format.
And just so it's said - this is not a reversible process! Jpeg only supports flat, 8 bit files with destructive compression, so the process permanently throws out a huge amount of data.
But even a large batch in Photoshop should only take about the time it takes to grab a cup of coffee in the meantime...? Why the extaordinary rush...?
Asking never hurts! Just in case if there's any such tool that can save time, I should have it! If not, photoshop is always the best
I am not sure if Irfanview will do it with PSD as the source file, but I suspect the at it will, as it can certainly view psd files.
As Dag says, it has to open each file, and takes about two or three seconds for each file. My gut has told me on occasions that I get a better result using Image Processor in Photoshop though, so I have not used Irfanview for a few years now.